Why i cant retrieve the date from database

Okay first of all, I have checked many questions on stackoverflow and those questions doesn't seems to solve my problem. Before some of you might put this question into a duplicate question, Can I have any explanation whats wrong with my code.

I want to calculate the days between two dates and the dates is being retrieved from database.
Below is what I have done

if($model->save()){
    $date1 = strtotime($startDate);
    $date2 = strtotime($endDate);
    
    $days = ($date2 - $date1);
    $Days = ($days/(60*60*24));
    $model->no_of_days= $Days;
    $model->save();
    
}

$startDate and $endDate are being retrieved from the database. When I code it like this, the result is 0.

but, when I replace it with a date, for example, '2020-10-10' and '2020-10-13', the result is 3.

My format for the date from the database is the same like the replaced date. Can someone guide me and explain to me.
What is wrong with my code?

Step One: I suggest to avoid using daysand Daysas variables.

Step Two: Verify the content of $startDate and $endDateby dumping them with the native var_dump function.

var_dump($startDate);
var_dump($endDate);

If data from your database are 'good', you should have the correct result. So we think that your $startDate and $endDate are not well initialized.

    $date1 = strtotime('2020-10-10');
    $date2 = strtotime('2020-10-13');
    $days = ($date2 - $date1);
    var_dump($days); //returns 259200
    $days = ($days/(60*60*24)); 
    var_dump($days); //returns 3

You code should be:

if($model->save()){
        $date1 = strtotime($startDate);
        $date2 = strtotime($endDate);

        $days = ($date2 - $date1);
        $days = ($days/(60*60*24));
        $model->no_of_days= $days;
        $model->save();

}

Step3:
I think you should have a look on DatetimeInterval class or date_diff function.

As you said,if the format of the replaced date is same as that of the date from the database you can try using the following:
Note : you are using $days and $Days variable. Use can use a single $days variable to store the date difference result and can use that variable for saving into database. I have changed using $Days into $days.

if($model->save()){
         // Calulating the difference in timestamps 
         $days = strtotime($endDate) - strtotime($startDate); 
        // 1 day = 24 hours 
        // 24 * 60 * 60 = 86400 seconds 
        $days= abs(round($days/ 86400)); 
        $model->no_of_days= $days;
        $model->save();

    }

Use date_diff

  $date1=date_create($startDate);
  $date2=date_create($endDate);
  $diff=date_diff($date1,$date2);
  echo $diff->format("%R%a");

If you pass correct $startDate and $endDate it should works correct.
Hope this help to you.